1. Field of the Invention
This invention relates to a torque sensor and a method for sensing torque using a measurement of strain correlated with the increase in coercive field of certain magnetostrictive, soft ferromagnetic materials upon application of stress.
2. Description of the Prior Art
Measurement of torque is frequently required in the design of industrial machinery. In order to determine critical design parameters within various elements, the designer must be able to measure accurately the mechanical forces, including torque, to which components may be subjected under operating conditions. Measurement of torque is also needed for the efficient operation of certain machines. For example, it may be desired for the sake of efficiency to operate an electric motor at a power level which causes as much torque to be applied to a load as the shaft connecting the motor to the load can tolerate without mechanical failure.
It is known in the art that a cylindrical shaft subjected to a mechanical torque undergoes a deformation which may be analyzed by the stresses therein. More particularly, the stresses in a shaft subjected only to torque are known to be purely tensile in the direction of one of the 45.degree. helices of the shaft, while the other 45.degree. helix, which has opposite chirality or handedness, exhibits purely compressional stress. A 45.degree. helix is defined conventionally as a locus of points of constant radius r from the axis of the cylinder such that points separated by a distance 1 when measured along the axis are at polar angles differing by 45 (1/r) degrees.
Several known sensors for measurement of torque in a cylindrical shaft rely on use of two strain gages adapted for the measurement of strain at the surface of the shaft. One of the strain gages is adapted to measure strain along a first 45.degree. helical direction. The other strain gage is adapted to measure strain along a second 45.degree. helical direction of chirality opposite that of the first 45.degree. helical direction. The torque to which the shaft is subjected may then be determined from the strains thus measured by formulae well known in the art.
A typical sensor of the type described above employs resistive strain gages composed of metal or semiconductor elements. The electrical resistance of such strain gages changes due to the elongation or compression of the active element (typically composed of wire or foil) due to the imparted stress The resistance is measured using well known electrical circuitry, frequently a Wheatstone bridge technique.
A major problem in use of such torque sensors to measure torque in a rotating shaft is the need to provide electrical connection to the shaft to supply energy to drive the strain gages and to transmit signals indicative of the strain therein. A number of methods have been employed to overcome this difficulty. Slip rings can provide a rotatable electrical connection between a rotating shaft and its fixed surroundings. Slip rings however are a source of electrical noise and are not fully reliable, especially at high rotation rates. Rotable transformers may be used only if AC excitation of the strain gages is acceptable. Generally, resistive strain measurement is less accurate using AC than DC excitation. Another alternative is use of drive circuitry and a telemetry transmitter mounted rigidly on the rotating shaft. Signals indicative of the strain being sensed by the strain gages are transmitted by radio from the shaft to external receiving means. Such circuitry is relatively bulky and complex.
A torque transducer employing stress-sensitive amorphous ribbon is disclosed by I. Sasada et al. (IEEE Trans. Magn. MAG-20, 951 (1984)). The transducer relies on a combination of shape anisotropy and anisotropy resulting from bending an amorphous ribbon around a shaft. An exciting coil and two sensing coils are used for the measurement of magnetic susceptibility (or equivalently of magnetic permebility) of two ribbons applied along +45.degree. and -45.degree. axes, the angles measured between the longitudinal direction of each ribbon and the shaft axis. The dynamic range is disclosed to be longer than 20 N-m for a 12 mm shaft. However, there is no disclosure in Sasada et al. of a torque transducer capable of measuring torques in excess of 100 N-m which are frequently encountered in industrial drive systems, for example. Furthermore, there is no disclosure in Sasada et al. of a torque transducer which measures torque by sensing a change in the magnetic coercivity of a magnetic element.
There remains a need in the art for torque sensors which are simple, reliable, and operationally independent of electrical connection to the shaft.